The invention relates to a method for the correction of unequal conversion characteristics during the processing of the signals from image sensors which are distributed in a planar arrangement. The invention also relates to an X-ray detector which is capable of carrying out such a method.
Image sensors which are distributed in a planar arrangement can be found in various applications such as, for example, electronic cameras. Hereinafter, flat dynamic X-ray detectors (FDXD) will be considered by way of example; such detectors typically comprise several thousands of individual image sensors (pixels) which are arranged in a matrix in the form of rows and columns. The primary image signals of the image sensors are read out in rows so that all image sensors of a column are temporarily successively addressed by the read-out circuitry. The primary image signals of a column, therefore, can be (successively) routed via the same processing unit. The processing units perform amplification and conversion of primary image signals, thus forming digital grey scale values (GW) as secondary image signals.
In the described X-ray detectors a problem is encountered in that each image sensor and each processing unit has an individual characteristic so that, despite the application of a uniform quantity of radiation to all image sensors, unequal secondary image signals may occur at the output of the X-ray detector. In order to correct such individual differences between the conversion characteristics, it is known from EP 1 081 942 A1 to assume a (multidimensional) linear map between the quantities of radiation actually incident on the image sensors and the secondary image signals (grey values) appearing at the output, which map is intended to describe the individual differences of the image sensors and the processing units. This linear image is determined by approximation from a plurality of calibration measurements. Its inverse can then be applied to real measured values so as to compensate the differences between the individual image sensors. The calibration measurement is carried out on the one hand by determining a dark image which represents the output signal without any radiation (“offset”). On the other hand, a so-called gain image is calculated which represents the individual gains of the individual combinations of an image sensor and a processing unit. However, it has been found that the correction that can be achieved by means of such a method is not enough to prevent the occurrence of disturbing artifacts, that is, notably in the case of special applications such as digital subtraction angiography (DSA).